Feedback system



May 31, 1949. L. s. NERGAARD FEEDBACK SYSTEM Filed Aug. 2, 1945 ANTENNA IN v ENTOR F/L. [EON S. MEG/JAE!) H54 7' N G Sol/RC5 w ATTORNEY Patented May 31, 1949 UNITED s'ms FEEDBACK SYSTEM Leon S. Nergaard, Princeton, N. 3., assignor to Radio Corporation of erica, a. corporation of Delaware 14 *Slaims.

This invention relates to a feedback system for vacuumtube circuits, and particularly to such a system when used in vacuum tube oscillators generating oscillations higher than 300 megacycles.

One difficulty which has been experienced in using-a push-pull vacuum tube grounded-grid oscillator for generating oscillations above 300 megacycles has been caused by flash-over at the operating power levels. It is believed that this flashover is caused by the high circulating currents and highvoltages in the feedback system.

In accordance with the present invention, this difficulty is overcome by usin a self-balancing feedback system. This self-balancing feedback system employs a single loop with a center tap ground connection having slight reactance at the operating frequency, thereby enabling the voltage nodal point on the loop to shift slightly to either side of the center tap, and permitting selfbalancing of the push-pull oscillation generator. The center tapping point itself, however, cannot departfrom ground potential to any appreciable extent.

Advantages of the self-balancing feedback system of the invention are: It is free from flashover at operating power levels because it appreciably increases the threshold value of flash-over, and it requires no tuning, thus eliminating the necessity for an additional control.

A more detailed description of the invention follows in conjunction with a drawing whose single figure shows a circuit diagram, partly schematic and partly elevational, of an ultra high frequency oscillation generator equipped with the feedback system of the invention.

Referring to the drawing, there is shown an ultra high frequency generator comprising a pair of vacuum tubes l and H mounted in and supported by a metallic plate l2. This metallic plate is provided with a pair of apertures l3, I3 for accommodating the envelopes of the vacuum tubes. The grids of the tubes It and H are directly connected to the metallic plate l2, which in turn is grounded. It will thus be seen that the grid of each tube and associated mounting electrostatically shields the anode and cathode and their associated circuits from each other. A tunable parallel conductor circuit composed of hollow conductors it connects the anodes of the two tubes together in push-pull relation. The ends of hollow conductors M at l5 are in telescopic relation to the other portions of the conductors in order to provide adjustment of the lengths of these conductors for tuning purposes. ..Putting it mother words, wehave. cs-

sentially a tubular conductor It: for tuning the anode circuit. Another pair of hollow conductors it forms a tunable circuit between the cathodes of the tubes. The conductors it are directly connected together by means of a short-circuiting disc l'i forming the bottom part i of an open metallic can-like arrangement 18. The filaments of the two tubes l0 and H are supplied with heating currents through wires extending within the interiors of the hollow conductors l6 and connected to the secondary winding of a suitable filament supply transformer l9 whose primary winding is connected to a low frequency alternating current filament heating source.

The anodes of the two tubes Ill and l l are supplied with a suitablepositive polarizing potential from a source +B through conductor 28 which is connected to the center of tubular conductor [5 and to the upper adjacent ends of the conductors M of the anode tuned circuit. The cathode tuning circuit is tunable by moving the can I8 up or down along the lengths of the conductors l6 by means of a suitable rod connected to plate ll. The anode and cathode tuned circuits l4 and I6, respectively, are separately shielded by metallic sheets shaped to form a cylindrical shield represented diagrammatically by means of the dash lines 2!. It will thus be seen that the anode and cathode tuned circuits are shielded from each other externally of the vacuum tubes l0 and H by shield and also shielded from each other electrostatically by means of the grounded grids Which are directly connected to the metallic disclike plate l2. The can l8, it should be noted, is open at the end nearest the cathodes. Can It is separated by an air gap from the shield 2i and is capacitively coupled to the shield.

In order to provide feedback between the anode and cathode circuits, there is provided a selfbalancing feedback circuit in the form of a loop 3D whose center or midpoint is coupled to the metallic ground plate 12 by means of a connection 38, 39. It will be seen that loop 33 extends above the metallic plate l2 through an aperture 32 while its legs 3| are below the metallic plate [2. The legs 3! of loop 3!! are electrically connected to the cathode line It, as shown. The, legs of the loop can, if desired, be telescoped to enable adjustment of their lengths, though this is not necessary if the feedback system is designed for a particular oscillator. The center-point ground connection 38, 39 comprises two lengths of conductors which are capacitively coupledto each-other cthrough mica insulation 33, 'thus grounding the midpoint of the loop over a path having slight reactance at the operating frequency. The mica 33, in effect, provides a series condenser in the connection between the center point of the loop and ground, Obviously any other arrangement of a series condenser will also function satisfactorily. The primary purpose of this condenser is to isolate the cathode circuit from ground so that a bias voltage may be applied between the grids and cathodes of the tubes. Because of this slight reactance in the center point ground connection, the system permits substantially perfect balance to be obtained in the currents through the two vacuum tubes despite slight diiferences in tube characteristics. The voltage nodal point on the loop 30 can shift slight- 1y to either side of the actual center point and thus permits self-balancing of the push-pull circuit. The center tap prevents flash-over at the operating power levels by holding the midpoint of the loop near ground potential and thus increasing the threshold value of flash-over.

The output circuit includes a metallic loop 3% which is arranged above the plate 52 for coupling to the anode tuned circuit. One end of the output loop 34 is connected directly to the plate 12 while the other end extends down through an aperture 35 for connection to the inner conductor of a concentric transmission line 38. Concentric transmission line 36, which connects the output loop 34 to a suitable utilization circuit such as an antenna, is shown provided with three impedance matching stubs 3?, 3i spaced one-quarter of a wavelength apart at the mid operating frequency. A cathode bias resistor ll! is connected between ground and that end of the cathode conductor it farthest removed from the vacuum tubes. The metallic can it which tunes the cathode tuned circuit serves as one terminal of a condenser whose other terminal comprises the grounded metallic shield 25.

The lengths of the anode and cathode tuned circuits, which are here shown to be electrically (although not necessarily mechanically) threequarters of a wavelength long, enable the attendant to tune the circuits at the extremely high frequencies at which the oscillator is capable of functioning.

The oscillator of the invention can be operated anywhere in the range from 350 to 1400 megacycles with anode voltages of positive polarity supplied to lead 2i] anywhere in the range from 300 volts to 15,000 volts. Obviously, if the circuit of the invention is to be operated continuously, the anode voltages would be somewhere near the 300 volt value, whereas, if the oscillation generator is to be used for pulse operation, for extremely short intervals of the order of a microsecond or so, the anode voltage may be in the upper regions near 15,000 volts. When using pulsing methods it is possible to obtain an output of at least 350 kilowatts employing an anode voltage of around 15,000 volts. Such pulse operation is obtainable by interrupting the anode voltage supply at the rate at which it is desired to produce pulses of high frequency energy.

By way of example only, the reactance of the condenser provided by the mica insulation 33 (separating the short conductors 38, 39) of the center point ground connection may be about 5 ohms at an operating frequency of 600 megacycles. This reactance, it will be noted, is greater than zero. The impedance of this center point ground connection 38, 39, 33 may be of the order of 20 to 100 ohms at operating frequencies in the 4 range from 485 to 715 megacycles. The value of the impedance of this center ground connection is not critical in this range. The optimum dimensions of the loop are best determined experimentally, that is, by trial and error. In one embodiment of the invention satisfactorily tried out in practice, the vacuum tubes In and II were RCA type 4033 power tubes which developed a power output of about 400 kw. for

. pulses of short duration, and the overall length of the center tap ground connection 38, 39 was constituted by a tube and a rod each about A" diameter having an overall length of about 5 cm.

It should be understood that the self-balancing feedback system of the invention with its center tap, as described, appreciably increases the threshold value of flash-over, but that it is still possible for undesired flash-over to occur if the power levels are unduly increased.

Although the invention has been described with particular reference to a single turn feed-back loop, it should be understood that it is not limited thereto since, if desired, a two turn feedback loop may be employed and still obtain the results of the invention. The use of a two-turn feedback loop provides a greater amount of feedback and believed to be preferred at the lower radio frequencies.

The term ground used in the specification and appended claims is deemed to include any o prising a vacuum tube having separate cathode and anode tuned circuits, and a feedback loop magnetically coupling both of said tuned circuits, and means for maintaining the center of said loop substantially at ground potential over a, path having reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency.

2. An electron discharge device system comprising a vacuum tube having cathode, anode and.

1 grid electrodes, individual tuned circuits coupled to said anode and cathode, a direct connection from said grid to ground, means for shielding said tuned circuits from each other, a feedback loop magnetically coupling both of said tuned circuits, and means for maintaining the center of said loop substantially at ground potential over a path having reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency.

3. An electron discharge device system comprising a pair of vacuum tubes each having an anode, a cathode and a grid, direct connections from the grids of said tubes to ground, a tuned circuit connected between said cathodes, a tuned circuit connected between said anodes, a feedback loop coupling said two tuned circuits together, and means for maintaining the center of said loop at substantially ground potential while permitting the voltage nodal point on the loop to shift from the physical center of the loop as a result of a difference in the characteristics of said two vacuum tubes.

l. A high frequency electron discharge device system comprising a pair of vacuum tubes each having an anode, a cathode and a grid, direct connections from the grids of said tubes to ground, a tuned circuit connected between said cathodes, a tuned circuit connected between said anodes, a feedback loop coupling said two tuned circuits together, and a connection having a reactance of the order of 20 to 100 ohms at the operating frequency coupling the center of said loop to ground.

5. A high frequency electron discharge device system comprising a pair of vacuum tubes each having an anode, a cathode and a grid, direct connections from the grids of said tubes to ground,

a tuned circuit connected between said cathodes,

a tuned circuit connected between said anodes, a feedback loop coupling said two tuned circuits together, and a connection having a reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency coupling the center of said loop to ground.

6. An ultra high frequency oscillator comprising a tube having a cathode, an anode and a grid, individual tunable resonant circuits coupled to said cathode and anode, means external of said tube for shielding said resonant circuits from each other, a direct conductive connection from said shield to said grid, a feedback loop magnetically coupling said tuned circuits together, the legs of said loop being connected to said cathode resonant circuit, and means for maintaining the center of said loop substantially at ground potential over a path having reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency.

7. An ultra high frequency oscillator comprising a vacuum tube having a cathode, an anode, and a grid, individual tunable resonant circuits coupled to said cathode and anode, means external of said tube for shielding said resonant circuits from each other, a direct conductive connection from said shield to said grid, a feedback loop magnetically coupling said tuned circuits together, and means for maintaining thecenter of said loop substantially at ground potential over a path having reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency.

8. An ultra high frequency oscillator comprising a vacuum tube having a cathode, an anode and a grid, said anode and cathode extending from opposite ends of the envelope of said tube, individual tuned circuits coupled to said anode and cathode extending on opposite sides of said tube, a shield intermediate said tuned circuits and connected to said grid, individual shields for said tuned circuits, a feedback circuit extending through an aperture in said first shield and feeding energy from the anode circuit to the cathode circuit, and a connection having a reactance in the range between 20 and 100 ohms at the operating frequency coupling the center of said feedback circuit to said first shield.

9. An ultra high frequency oscillator comprising a vacuum tube having a cathode, an anode, and a grid, said anode and cathode extending from opposite ends of the envelope of said tube,

individual tuned circuits coupled to said anode and cathode extending on opposite sides of said tube, said individual tuned circuits having substantially uniformly distributed constants and being each an odd multiple greater than unity of one-quarter of a wavelength at the operating frequency, a shield intermediate said tuned circuits and connected to said grid, individual shields for said tuned circuits, a feedback loop extending through an aperture in said first shield and feeding energy from the anode circuit to the cathode circuit, and means for maintaining the center of said loop substantially at ground potential over a path having reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency.

10. An ultra high frequency oscillator comprising a pair of vacuum tubes each having an anode, a cathode and a grid, a parallel conductor tuned circuit of substantially uniformly distributed constants connecting said anodes together, a parallel conductor tuned circuit of substantially uniformly distributed constants connecting said cathodes together, a connection from ground to each of said grids to electrostatically shield the anode and cathode of each tube from each other, means external of said tubes for shielding said tuned circuits from each other, and a feedback circuit coupling said two tuned circuits, said feedback circuit including a loop whose legs are connected to the cathode tuned circuit, and means for maintaining the center of said loop at substantially ground potential while permitting the voltage point on the loop to shift from the physical center of the loop as a result of any differences in the characteristics of said pair of'vacuum tubes, said last means including a condenser coupling said loop at substantially the midpoint to ground.

11. An electron discharge device system comprising a vacuum tube having separate cathode and anode tuned circuits, and a single turn feedback loop magnetically coupling both of said tuned circuits, and means for maintaining the center of said loop substantially at ground potential over a path having reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency.

12. An electron discharge device system comprising a vacuum tube having separate cathode and anode tuned circuits, and a feedback loop magnetically coupling both of said tuned circuits, means for maintaining the center of said loop substantially at ground potential over a path having reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency, the ends of said loop being tightly coupled to said cathode circuit.

13. An electron discharge device system comprising a vacuum tube having separate cathode and anode tuned circuits, and a feedback loop magnetically coupling both of said tuned circuits, and a condenser directly coupling the center of said loop to ground, said condenser having reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency.

14. An electron discharge device system comprising a vacuum tube having separate cathode and anode tuned circuits, and a feedback loop magnetically coupling both of said tuned circuits, and a condenser directly coupling the center of said loop to ground, said condenser having reactance which is appreciably greater than zero but low compared to the reactance of said loop at the operating frequency, and means directly connecting the ends of said loop to said cathode tuned circuit.

LEON S. NERGAARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Lawrence July 9, 1946 Number 

